This invention relates to a process for producing xylylene diisocyanate by thermal decomposition of xylylene dicarbamate (which will be hereinafter referred to as XDC) as an urethane compound. Xylelene diisocyanate is a diisocyanate having a high reactivity and a good yellowing resistance, which is useful as a raw material for polyurethane and polyurea.
Xylylene diisocyanate is now commercially produced only by reaction of xylylenediamine with phosgene, that is, by phosgenation process, and no other processes have been commercially practised. The phosgenation process has problems in the handling of toxic phosgene, treatment of by-product hydrogen chloride and corrosion of reactor. Thus, development of another commercial process for producing xylylene diisocyanate has been desired as a substitute.
A process for producing xylylene diisocyanate by thermal decomposition of XDC as an urethane compound is available as a process for synthesizing xylylene diisocyanate without using phosgene. Thermal decomposition of XDC as an urethane compound proceeds even in the absence of a catalyst to produce xylylene diisocyanate, but its space-time yield is low owing to low selectivity and reaction rate.
It is also known that an isocyanate and an alcohol can be obtained by thermal decomposition of an urethane compound according to either gas phase process or liquid phase process. The gas phase process belongs to high temperature reaction with much heat of endothermic reaction. This leads to a difficult design of reactors and consequent poor practicality. On the other hand, various liquid phase processes have been proposed. In order to accelerate the thermal decomposition rate and obtain a polymerizable isocyanate with a higher selectivity in a process for producing an isocyanate compound by thermal decomposition of an urethane compound, it is proposed to use a catalyst or to use a carrier for preventing recombination with alcohol.
For example, British Patent No. 2113673A discloses a process for producing an aromatic isocyanate by thermal decomposition of an urethane compound in the presence of a catalyst containing Ti, Sb, Zr or Sn under an atmospheric pressure or superatmospheric pressure (not reduced pressure), but conversion to isocyanate is found to be very poor.
Japanese Patent Application Kokai (Laid-open) No. 54-88201 discloses use of alkaline earth metals such as Be, Mg, Ca, Ba, Sr and Ra or their compounds as a thermal decomposition catalyst.
EPO No. 323514 discloses a process for producing an aliphatic isocyanate by using a simple substance selected from Mn, Mo, W and Zn or their compounds as a thermal decomposition catalyst.
U.S. Pat. No. 4,873,365 discloses a process for producing an alicyclic isophorone diisocyanate by using SnO.sub.2 or CuO, or their mixture in the absence of a solvent.
Japanese Patent Application Kokai (Laid-open) No. 57-158747 discloses a process using a catalyst containing at least one of elements selected from copper group, zinc group, aluminum group, carbon group except carbon, and titanium group, and compounds selected from oxides and sulfides of these elements, and also using an inert gas or a low boiling organic solvent as a carrier for preventing recombination of the resulting isocyanate with the resulting alcohol.
As a result of extensive studies on thermal decomposition of XDC as an urethane compound in the above-mentioned conventional processes, the present inventors have found that acceleration of thermal decomposition takes place in some cases, depending on the kind of catalysts used, and furthermore side reactions such as polymerization reaction is increased at the same time, and thus it is difficult to obtain xylylene diisocyanate at a higher decomposition rate with a higher selectivity in the conventional processes, because it seems that xylylene diisocyanate structurally has both aromatic and aliphatic properties and is particularly more polymerizable among a large number of isocyanates (see British Patent No. 1192859 page 2, lines 67-85).